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ISIS sympathizers inside U.S. a growing concern

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Officials are concerned about plots from growing number of U.S.-based ISIS sympathizers

ISIS uses social media as well as encrypted online communications to spread message

Assistant Attorney General: "What we're seeing is unprecedented"

(CNN)Hours before his fatal encounter with anti-terrorism officers, Usaamah Rahim told an associate he was switching from plans to behead a conservative blogger to assaults on the "boys in blue."

Rahim, who officials believe was radicalized by ISIS, referred to his planned act of jihad against police officers in coded language -- "going on vacation," he said, according to an FBI affidavit.

The 26-year-old security guard's so-called vacation ended in a hail of bullets Tuesday. FBI and Boston police officers tailing him suspected he was about to launch an attack. An associate, David Wright, 25, was arrested and charged with obstruction.

The investigation into the depth of Rahim's network and possible overseas connections comes amid concerns by counterterrorism officials about future plots from a growing number of U.S.-based ISIS sympathizers.

Using social media as well as encrypted online communications beyond the reach of law enforcement surveillance, the terror organization is increasingly reaching new sympathizers and encouraging attacks such as the one foiled in Boston, officials said.

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"The foreign terrorist now has direct access into the United States like never before," Michael Steinbach, assistant director of the FBI's counterterrorism division, told the House Homeland Security Committee this week.

At least one of the men connected to the plot to behead conservative blogger Pamela Geller and kill officers in Massachusetts was being encouraged online by people overseas connected to ISIS, two U.S. officials with knowledge of the investigation said Thursday. The officials, however, cautioned that the exact ties are difficult to know.

'Thousands of messages'

Rep. Michael McCaul, R-Texas, chairman of the Homeland Security Committee, said communications with ISIS first brought Rahim to the attention of law enforcement.

"He was being investigated by the Boston Joint Terrorism Task Force after communicating with and spreading ISIS propaganda online," McCaul said.

An estimated 3,400 Westerners have traveled to join ISIS in its quest to establish an Islamist state in Iraq and Syria, according to counterterrorism officials. At least 200 Americans have gone or attempted to travel to Syria. But even a rough estimate of the number of sympathizers on U.S. soil is hard to discern.

"There are thousands of messages being put out into the ethersphere and they're just hoping that they land on an individual who's susceptible to that type of terrorist propaganda," said John Carlin, the assistant attorney general heading the Justice Department's national-security division.

"They just need to be right once to get a terrorist attack inside the United States."

ISIS has the most sophisticated propaganda machine of any terrorist organization, a global communications strategy that has stumped counterterrorism officials while making significant inroads among U.S. sympathizers.

"How many of those followers are actually in the United States, in your estimate?" McCaul asked at this week's hearing.

"There's hundreds, maybe thousands," Steinbach replied. "It's a challenge to get a full understanding of just how many of those passive followers are taking action."

'A call to arms'

Rahim was "active on social media in ways that sparked interest" from law enforcement, according to a congressional source briefed on the investigation. His online interactions with possible ISIS contacts were tantamount to what the source called a "conspiracy over the Internet."

While there was an effort to motivate action by Rahim, two U.S. officials told CNN they, so far, don't believe ISIS was directly involved in planning or directing the attack.

Authorities described a scenario similar to the attempted ambush last month of a Garland, Texas, event featuring controversial cartoons of the Muslim Prophet Mohammed. The two attackers were killed by an off-duty police officer; one of the men was said to have been in contact with ISIS.

Geller, president of the American Freedom Defense Initiative, which includes subsidiary programs Stop Islamization of America and Stop Islamization of Nations, organized the Texas event. She was the original target of this week's plot.

"The targeting of the Mohammed Art Exhibit and Contest exemplifies the call to arms approach encouraged by (ISIS) along with the power of viral messaging," Steinbach said. "In this instance, the event gained much publicity prior to it occurring and attracted negative attention that reached areas of the country -- and the world -- that it may not have without the widespread reach of the Internet."

Steinbach said the targeting of U.S. military personnel by ISIS supporters was evident in the recent release of hundreds of names of U.S. military personnel. The names were posted to the Internet and quickly spread via social media.

"Once they identify an individual, they'll then try to directly communicate with that individual to give them coaching and guidance on how to do an attack inside the United States," Carlin said.

'Lone actor attack'

ISIS recruitment efforts have become more difficult to detect because of encrypted "dark space" communication, officials said.

"Do we have any idea how many communications are taking place in the dark space?" McCaul asked.

"No, we don't and that's the problem -- the ability to know what they're saying in these encrypted communication situations is troubling," Steinbach said.

ISIS uses its social media prowess to lure more and more Americans, who are often young, sometimes disillusioned.

"What they're telling them is, here's some easily available -- readily available information online that you can exploit," said John Mulligan, deputy director of the National Counterterrorism Center. "In other words, they believe that they can provide them everything that they will need to undertake some kind of lone actor attack."

In January, the New York Police Department and other law enforcement agencies responded to a threat from ISIS after the re-released of a September 2014 online message and video urging followers to "rise up and kill intelligence officers, police officers, soldiers and civilians."

The threat named the United States, France, Australia and Canada as targets.